Copying others appears to be a cost-effective way of obtaining adaptive information, particularly when flexibly employed. However, adult humans differ considerably in their propensity to use information from others, even when this 'social information' is beneficial, raising the possibility that stable individual differences constrain flexibility in social information use. We used two dissimilar decision-making computer games to investigate whether individuals flexibly adjusted their use of social information to current conditions or whether they valued social information similarly in both games. Participants also completed established personality questionnaires. We found that participants demonstrated considerable flexibility, adjusting social information use to current conditions. In particular, individuals employed a 'copy-when-uncertain' social learning strategy, supporting a core, but untested, assumption of influential theoretical models of cultural transmission. Moreover, participants adjusted the amount invested in their decision based on the perceived reliability of personally gathered information combined with the available social information. However, despite this strategic flexibility, participants also exhibited consistent individual differences in their propensities to use and value social information. Moreover, individuals who favoured social information self-reported as more collectivist than others. We discuss the implications of our results for social information use and cultural transmission.

Dopamine neurons in the ventral tegmental area (VTA) are implicated in affective functions. However, it is unclear to what extent dopamine neurons in substantia nigra pars compacta (SNc) play such roles. TH-Cre transgenic mice received adeno-associated viral vectors encoding channelrhodopsin2 (ChR2), halorhodopsin (NpHR), or control vector into the VTA or SNc, resulting in selective expression of these opsins in dopamine neurons. Mice with ChR2 learned instrumental responding to deliver photostimulation into the VTA or SNc and also sought for the compartment where they received photostimulation (i.e., operant place preference). Operant place preference scores were highly correlated with self-stimulation responses. In contrast, mice with NpHR avoided the compartment where they received photostimulation into the VTA, SNc, or dorsal striatum, whereas control mice did not. These observations suggest that the excitation and inhibition of SNc dopamine neurons elicit positive and negative affective effects, respectively, similar to those of VTA dopamine neurons.

The dorsal anterior cingulate cortex (dACC) is a key hub of the brain's executive control system. Although a great deal is known about its role in outcome monitoring and behavioral adjustment, whether and how it contributes to the decision process remain unclear. Some theories suggest that dACC neurons track decision variables (e.g., option values) that feed into choice processes and is thus “predecisional.” Other theories suggest that dACC activity patterns differ qualitatively depending on the choice that is made and is thus “postdecisional.” To compare these hypotheses, we examined responses of 124 dACC neurons in a simple foraging task in which monkeys accepted or rejected offers of delayed rewards. In this task, options that vary in benefit (reward size) and cost (delay) appear for 1 s; accepting the option provides the cued reward after the cued delay. To get at dACC neurons' contributions to decisions, we focused on responses around the time of choice, several seconds before the reward and the end of the trial. We found that dACC neurons signal the foregone value of the rejected option, a postdecisional variable. Neurons also signal the profitability (that is, the relative value) of the offer, but even these signals are qualitatively different on accept and reject decisions, meaning that they are also postdecisional. These results suggest that dACC can be placed late in the decision process and also support models that give it a regulatory role in decision, rather than serving as a site of comparison.

Making predictions about the rewards associated with environmental stimuli and updating those predictions through feedback is an essential aspect of adaptive behavior. Theorists have argued that dopamine encodes a reward prediction error (RPE) signal that is used in such a reinforcement learning process. Recent work with fMRI has demonstrated that the BOLD signal in dopaminergic target areas meets both necessary and sufficient conditions of an axiomatic model of the RPE hypothesis. However, there has been no direct evidence that dopamine release itself also meets necessary and sufficient criteria for encoding an RPE signal. Further, the fact that dopamine neurons have low tonic firing rates that yield a limited dynamic range for encoding negative RPEs has led to significant debate about whether positive and negative prediction errors are encoded on a similar scale. To address both of these issues, we used fast-scan cyclic voltammetry to measure reward-evoked dopamine release at carbon fiber electrodes chronically implanted in the nucleus accumbens core of rats trained on a probabilistic decision-making task. We demonstrate that dopamine concentrations transmit a bidirectional RPE signal with symmetrical encoding of positive and negative RPEs. Our findings strengthen the case that changes in dopamine concentration alone are sufficient to encode the full range of RPEs necessary for reinforcement learning.

Bacteria frequently live in densely populated surface-bound communities, termed biofilms [1,2,3,4]. Biofilm-dwelling cells rely on secretion of extracellular substances to construct their communities and to capture nutrients from the environment [5]. Some secreted factors behave as cooperative public goods: they can be exploited by nonproducing cells [6,7,8,9,10,11]. The means by which public-good-producing bacteria avert exploitation in biofilm environments are largely unknown. Using experiments with Vibrio cholerae, which secretes extracellular enzymes to digest its primary food source, the solid polymer chitin, we show that the public goods dilemma may be solved by two very different mechanisms: cells can produce thick biofilms that confine the goods to producers, or fluid flow can remove soluble products of chitin digestion, denying access to nonproducers. Both processes are unified by limiting the distance over which enzyme-secreting cells provide benefits to neighbors, resulting in preferential benefit to nearby clonemates and allowing kin selection to favor public good production. Our results demonstrate new mechanisms by which the physical conditions of natural habitats can interact with bacterial physiology to promote the evolution of cooperation.

Optogenetics is a recently developed method in which neurons are genetically modified to express membrane proteins sensitive to light, enabling precisely targeted control of neural activity [1,2,3]. The temporal and spatial precision afforded by neural stimulation by light holds promise as a powerful alternative to current methods of neural control, which rely predominantly on electrical and pharmacological methods, in both research and clinical settings [4,5]. Although the optogenetic approach has been widely used in rodent and other small animal models to study neural circuitry [6,7,8], its functional application in primate models has proven more difficult. In contrast to the relatively large literature on the effects of cortical electrical microstimulation in perceptual and decision-making tasks [9,10,11,12,13], previous studies of optogenetic stimulation in primates have not demonstrated its utility in similar paradigms [14,15,16,17,18]. In this study, we directly compare the effects of optogenetic activation and electrical microstimulation in the lateral intraparietal area during a visuospatial discrimination task. We observed significant and predictable biases in visual attention in response to both forms of stimulation that are consistent with the experimental modulation of a visual salience map. Our results demonstrate the power of optogenetics as a viable alternative to electrical microstimulation for the precise dissection of the cortical pathways of high-level processes in the primate brain.

Adaptive behaviors increase the likelihood of survival and reproduction and improve the quality of life. However, it is often difficult to identify optimal behaviors in real life due to the complexity of the decision maker's environment and social dynamics. As a result, although many different brain areas and circuits are involved in decision making, evolutionary and learning solutions adopted by individual decision makers sometimes produce suboptimal outcomes. Although these problems are exacerbated in numerous neurological and psychiatric disorders, their underlying neurobiological causes remain incompletely understood. In this review, theoretical frameworks in economics and machine learning and their applications in recent behavioral and neurobiological studies are summarized. Examples of such applications in clinical domains are also discussed for substance abuse, Parkinson's disease, attention-deficit/hyperactivity disorder, schizophrenia, mood disorders, and autism. Findings from these studies have begun to lay the foundations necessary to improve diagnostics and treatment for various neurological and psychiatric disorders.

2014年1月6日月曜日

Comparative neuroimaging can identify unique features of the human brain and teach us about human brain evolution. Comparisons with chimpanzees, our closest living primate relative, are critical in this endeavor. Structural magnetic resonance imaging (MRI) has been used to compare brain size development, brain structure proportions and brain aging. Positron emission tomography (PET) imaging has been used to compare resting brain glucose metabolism. Functional MRI (fMRI) has been used to compare auditory and visual system pathways, as well as resting-state networks of connectivity. Finally, diffusion-weighted imaging (DWI) has been used to compare structural connectivity. Collectively, these methods have revealed human brain specializations with respect to development, cortical organization, connectivity, and aging. These findings inform our knowledge of the evolutionary changes responsible for the special features of the modern human mind.

The central nervous system rapidly learns that particular stimuli predict imminent danger. This learning is thought to involve associations between neutral and harmful stimuli in cortical and limbic brain regions, though associative neuroplasticity in sensory structures is increasingly appreciated. We observed the synaptic output of olfactory sensory neurons (OSNs) in individual mice before and after they learned that a particular odor indicated an impending foot shock. OSNs are the first cells in the olfactory system, physically contacting the odor molecules in the nose and projecting their axons to the brain’s olfactory bulb. OSN output evoked by the shock-predictive odor was selectively facilitated after fear conditioning. These results indicate that affective information about a stimulus can be encoded in its very earliest representation in the nervous system.